The present invention relates to supplying refrigerant to a miniature open-circuit refrigerator. In this connection, the invention relates on the one hand to a method of supplying refrigerant to such a refrigerator, and on the other to any refrigeration apparatus which includes an open-circuit refrigerator making use of the said method.
The refrigerators considered in the context of the present invention are capable of employing refrigeration cycles, such as the Joule-Thompson cycle, in which cooling energy is produced by the isenthalpic expansion of a working refrigerant fluid.
Such refrigerators operate by the expansion of a working refrigerant fluid, that is to say they employ either at least one isenthalpic expansion or at least one expansion which is of both an isenthalpic and isentropic nature.
A refrigerator contains a working circuit which has on the one hand an inlet for the working refrigerant fluid, which is intended to be connected, by means of a connecting valve for example, to a reservoir for supplying refrigerant fluid at high pressure, and on the other hand an outlet for the said refrigerant fluid once it has expanded, which outlet communicates freely with the atmosphere outside the refrigerator, or with a receptacle for recovering the expanded refrigerant fluid.
The present invention is involved in broad terms with the starting-up phase of the refrigerators defined above and will now be illustrated by reference to an open-circuit refrigerator of the Joule-Thompson type.
By the "starting-up phase" of a refrigerator is meant, by contrast with the working phase proper of the refrigerator, that brief period of operation during which, simply by the refrigerator being put into operation, the cold temperature or temperatures generated alter and drop from an initial level close to the ambient temperature around the refrigerator to a final level substantially equal to the rated cold temperature or temperatures which the aforesaid refrigerator is designed, calculated and dimensioned to generate.
Consequently, "working phase" thus means the period during which the refrigerator is in stable and steady operation and which immediately succeeds the starting-up phase defined above, and during which the cold temperature or temperatures generated remain steady and equal to the rated cold temperature level defined above.
In general terms, open-circuit refrigerators of the Joule-Thompson type comprise:
a heat-exchanger which has on the one hand a first duct for the working refrigerant fluid, which is at a high pressure, and on the other hand a second duct for the expanded refrigerant fluid, which is at a low pressure, the first and second ducts being in a heat-exchanging relationship one with the other,
a member for isenthalpic pressure release, such as a calibrated orifice, whose upstream end communicates with the said first duct,
a chamber for expanding the refrigerant fluid to the low pressure, and in particular for collecting the said fluid, the fluid possibly being at least partly condensed following the said expansion. This chamber communicates with the downstream end of the said pressure-release member and with the second duct of the said heat-exchanger. It is in this expansion chamber that the cooling energy produced by the refrigerator becomes available.
In certain cases such refrigerators also include a means of regulating the cooling energy produced, in which case the following are provided, generally speaking:
a pressure-release member capable of regulating the throughput of expanded refrigerant fluid, which has on the one hand a seating provided with an expansion orifice, and on the other a needle-valve which, in conjunction with the said orifice, defines a pressure-release passage for the working refrigerant fluid, one of these two members (the seating and the needle-valve) being movable relative to the other, which is fixed.
a direct-acting regulating means which consists of a temperature-sensitive regulating container holding a charge of a fluid capable of expanding under the effect of temperature, at least a part of which is in heat-exchanging relationship with at least the second duct from the said heat exchanger. This container is at least partly bounded by a bellows of which one end is fixed and the other is movable, with the movable end controlling the movement of the movable part of the pressure-release member as a function of the temperature reached in the said regulating container.
For certain applications in which miniature refrigerators of the Joule-Thompson type are used the duration of the starting-up phase is too long even when it lasts only something of the order of ten seconds.
In general, as in this particular case, the length of the starting-up phase depends chiefly on:
the total amount of metal in the refrigerator. The larger this amount the greater the thermal inertia of the refrigerator and the longer the starting-up phase.
the mean cooling energy produced by the refrigerator during the starting-up phase which is generated by the isenthalpic expansion of the working refrigerant fluid. The greater this energy the shorter the starting-up phase.
These are the two chief parameters on which the length of the starting-up phase depends. In effect, for a given low pressure representing the pressure to which the refrigerant fluid is expanded during the working phase, which may be atmospheric pressure for example, the nature of the said refrigerant fluid is selected as a function of its own boiling point at the above-mentioned low pressure and to suit the rated cold temperature level or levels which the refrigerator is required to generate. Consequently, the nature of the working refrigerant fluid is selected once and for all as a function of the design characteristics of the refrigerator.
By reducing the amount of metal in the refrigerator it is thus possible, in theory, to shorten the starting-up phase. In fact, this amount of metal cannot be reduced to any major degree in practice without having a substantial effect on the effectiveness and reliability of the refrigerator. Thus, the first duct of the heat-exchanger of an open-circuit Joule-Thompson refrigerator generally consists of a relatively thick coiled tube, given the relatively high working pressure of the working refrigerant fluid while the refrigerator is starting-up, which may be of the order of 400 bars for example. The thickness of the tube cannot normally be reduced below a certain figure without a danger of the said coiled tube rupturing.
To increase the mean cooling energy produced by the refrigerator during the starting-up phase with a selected working refrigerant fluid, it is possible:
either to increase the throughput of working refrigerant fluid which is expanded during the starting-up phase,
or to increase the ratio in which the working refrigerant fluid is expanded at the pressure-release member of the refrigerator.
In the first case, if it is desired to bring about an automatic increase in the throughput of refrigerant fluid during the starting-up phase, this entails adding to the refrigerator either a regulating means similar to the one described above, i.e. a means which consists of a temperature-sensitive regulating container, or else a second high-pressure supply duct which takes the place of or is connected in parallel with the first duct during the starting-up or cooling-down phase. This however involves a commensurate increase in the mass of the refrigerator and thus in its thermal inertia and in this way part of the benefit gained from the larger expansion throughout is lost.
In the second case, since the low operating pressure of the refrigerator is generally designed to be equal to atmospheric pressure, the ratio of expansion at the pressure-release member can be increased only by raising the high pressure of the working refrigerant fluid, i.e. by increasing the pressure in the reservoir which supplies the refrigerant fluid. This however in turn entails a considerable increase in the thickness of the walls of the said reservoir and/or the use of materials of high mechanical strength. This being the case the supply reservoir becomes a very expensive piece of equipment.
The present invention thus has as an object to enable an open-circuit refrigerator, in particular one of the Joule-Thompson type, to be started-up quickly, and for this to be possible without affecting the design of the refrigerator used, that is to say without any substantial changes to its structure and operation.